1. Field of the Invention
The present invention relates to a nonreducing dielectric ceramic composition and, more particularly, to a composition used as a dielectric material for monolithic capacitors which include a base metal, such as nickel, as an internal electrode material.
2. Description of the Prior Art
Conventional dielectric ceramic materials are reduced to a semiconductive material when fired under neutral or reducing low partial pressure of oxygen. Therefore, a noble metal such as Pd or Pt should be used as an internal electrode material because they do not melt at sintering temperatures of the dielectric ceramic material and are not oxidized with the dielectric ceramic materials even if the sintering is carried out with a high partial pressure of oxygen. However, the use of such a noble metal is an obstacle to reduction of the manufacturing costs of monolithic ceramic capacitors.
Thus, in order to solve the above problem, it is desired to use a base metal such as Ni as an internal electrode material. However, when using such a base metal as the internal electrode material, it oxidizes and does not function as an electrode. Therefore, in order to use such a base metal as the internal electrode material, a dielectric ceramic material must be used which does not reduce to a semiconductive material even when fired in a neutral or reducing atmospheric gas having a low oxygen partial pressure but yet has a sufficient resistivity and a superior dielectric property. A dielectric ceramic material which meets these requirements is, for example, a BaTiO.sub.3 -CaZrO.sub.3 -MnO-MgO-based composition disclosed in Japanese patent Application Laid-Open No. 62-256422 (1987), or a BaTiO.sub.3 -- (Mg, Zn, Sr, Ca)O-B.sub.2 O.sub.3 -SiO.sub.2 -based composition disclosed in Japanese patent publication No. 61-14611 (1986).
However, in the nonreducing dielectric ceramic composition disclosed in Japanese Patent Application Laid Open No. 62-256422 (1987), CaZrO.sub.3 or CaTiO.sub.3 produced in the firing process is apt to provide a secondary phase with Mn or the like, and thus this would lead to danger of impairing reliability of the composition in a high temperature range.
Also, the composition disclosed in Japanese Patent Publication No. 61-14611 (1986) has dielectric constants of 1,000-2,800, and these values are inferior to the dielectric constants of 3,000-3,500 of the conventional ceramic composition using a noble metal such as Pd. Therefore, it is difficult to produce ceramic capacitors with a large capacitance without an increase in volume.
Still further, the temperature change rate of a dielectric constant of this composition falls within a change rate of .+-.10% between -25.degree. C. and +85.degree. C. with respect to a dielectric constant at 20.degree. C., but in a change rate of high temperature exceeding +85.degree. C., the change rate exceeds 10% and greatly shifts from the X7R characteristics specific by EIA.